1. Field of Invention:
This invention relates generally to oscillatory flowmeters in which a jet stream of incoming fluid is periodically switched in the interaction zone of a steering chamber from one guide to another to generate fluidic oscillations therein which are sensed to generate an output signal, and more particularly, to a meter of this type capable of measuring both mass flow and volumetric flow.
2. State of Art:
As pointed out by Wilson et al. in their article "Experimental Investigation of a Fluidic Volume Flowmeter" in the March 1970 issue of the Journal of Basic Engineering (ASME), the most important characteristic of a good flowmeter is an output signal that is linearly related to the quantity being measured. Other features found only in good flowmeters are reliability, and the absence of moving parts that can be jammed with debris. The accuracy of a good flowmeter should be unaffected by changes in the properties of the fluid being metered and by variations in ambient conditions, such as temperature and pressure.
Because no one available type of flowmeter satisfies all of the requisite features and characteristics of a good flowmeter, existing flowmeters fall short of many industrial needs and the search for new flow-metering techniques remains a continuing process. The Wilson et al. article deals with a new type of oscillatory flowmeter that possesses many of the features one looks for in a commercially-acceptable meter.
A more detailed description of an oscillatory flowmeter appears in the 1972 Adams U.S. Pat. No. 3,640,133. In the Adams meter, incoming fluid is fed as a jet stream into a steering chamber through a power nozzle, the chamber being provided with an output duct which is in line with the power nozzle. Disposed within the steering chamber are twin guides which diverge from the power nozzle. These guides are spaced from each other to define an interaction zone and are spaced from the opposing sidewalls of the chamber to define feedback paths. Each path has an inlet at the downstream end of the related guide and an outlet at the upstream end which functions as a control nozzle.
The Adams meter exploits the Coanada effect; that is, the natural tendency of a fluid jet to follow the contour of a wall when the jet is discharged adjacent to the wall surface, even when this surface is curved away from the jet discharge axis. In the Adams meter, the fluid jet emitted from the power nozzle attaches itself to the wall of one of the diverging guides and is conducted downstream thereby into the inlet of the associated feedback path which leads the fluid to the outlet control nozzle. The fluid projected from the control nozzle into the interaction zone between the guides acts to deflect the jet emitted from the power nozzle toward the wall of the other guide where the same hydraulic action is repeated.
As a consequence, the jet in the steering chamber of the Adams meter switches periodically from guide to guide to generate fluidic oscillations whose frequency is proportional to the volumetric flow rate. In this oscillatory flowmeter, the relationship is the same for all liquids and gases in the turbulent range, and the calibration of the flowmeter is therefore independent of the properties of the fluid.
In order to convert the fluidic oscillations into an electrical signal having a corresponding frequency, Adams inserts a sensing probe into the steering chamber between the guides, the probe being mounted on a bellows and being caused to vibrate at a rate in accordance with the frequency of the oscillating fluid. This probe is operatively coupled to the movable core of a variable transformer external to the steering chamber.
The Adams sensor arrangement is relatively insensitive and performs poorly at low flow rates. And because the sensor is of the vibratory type, it is also responsive to externally-generated vibratory forces and therefore exhibits a poor signal-to-noise ratio. Moreover, the Adams meter only generates a signal that is indicative of flow rate, and yields no signal as a function of mass flow. In industrial flow measurement, it is often necessary to provide a reading of mass flow--that is, the mass of fluid flowing past or through a reference plane per unit of time.
Vortex type meters, such as the meters disclosed in my prior Herzl U.S. Pat. Nos. 3,776,033 and 4,010,645, are capable of measuring mass flow as well as volumetric flow of fluids being treated or supplied in industrial processes. However, such meters which make use of a vortex shedding body placed in the flow stream are unreliable and inaccurate below pipe Reynolds numbers of 5000, and they are non-functional below pipe Reynolds numbers of 3000. Hence, these meters cannot be made in pipe sizes of less than about 1 inch.